This research study evaluates whether children and adolescents with sickle cell disease have problems with breathing and oxygen levels when they sleep. During sleep, everyone has lower oxygen levels in the blood than when they are awake. However, this decrease is not significant if one has normal lungs. In persons with sickle cell disease, low levels of oxygen during sleep could lead to problems like more frequent or severe pain crises. In some patients, the detection of obstructive sleep apnea and sleeping hypxemia may help in the timely institution of appropriate therapy.